Secretary of State John Kerry arrived in Seoul today on a mission aimed at tempering rising tensions on the Korean peninsula, urging North Korean leader Kim Jong-un to opt for "the peaceful option being offered."

His four day trip to East Asia, which includes stops in China and Japan, came as a wary region braced for a potential missile launch by Pyongyang, for yet another day.

Kerry met with South Korean President Park Geun-Hye, reassuring the new leader the U.S. would defend its ally, and added North Korea's bellicose rhetoric was "simply unacceptable."

In a sign the U.S. was making efforts to tone down its own rhetoric, Kerry stressed the two discussed a "bright vision of possibilities" and the prospect of a reunited Korean peninsula "where the aspirations of two Koreas are being made," he said.

"We want to emphasize that the real goal is not reinforcing that we will defend our allies, but the possibilities of peace, the possibilities of reunification," Kerry said at a joint press conference with South Korean Foreign Minister Yun Byung-se.

Kerry's visit comes on the heels of an alarming assessment from the Defense Intelligence Agency, expressing "moderate confidence" North Korea might have a nuclear weapon small enough to be placed on a ballistic missile. Sources tell ABC News that sentence, buried deep in a classified report, was accidentally declassified, catching intelligence agencies off guard when Republican Congressman Doug Lamborn asked the Joint Chiefs Chairman about it at a House Armed Services Committee hearing Thursday.